The present invention relates generally to ceramic coatings for use in gas turbine engines, and more particularly to ceramic coatings which provide sealing and thermal insulation.
Gas turbine engines are well known sources of motive power, e.g., for aircraft, and generally include compressor, combustor and turbine sections. As illustrated generally in FIG. 1, compressor and turbine sections each include shaft-mounted, rotating disks 1, each carrying a set of blades 2 located within a hollow housing or case 3, with intervening sets of stationary vanes 5 mounted to the case. Air seals 4, 7 are provided between the tips of the blades and the case, and between the vanes and the disks to prevent air leakage between those components.
Air is ingested through an engine inlet and compressed by rotating disks and associated blades in the compressor. The compressed air is then burned with fuel in the combustor to generate high pressure and temperature gasses, which cause rotation of the turbine sections and associated fan compressor stages and are then ejected out an engine exhaust to provide thrust. The case is intended to prevent leakage of air or combustion products around the tips of the blades, i.e., between the blade tips and the case, which leakage reduces the efficiency of the engine.
Despite the design of components to minimize leakage, a substantial proportion of any leakage which does occur in a normally-operating gas turbine engine occurs between the tips of the blades and the case, and between the tips of the vanes and the disks. One manner of eliminating such leakage is to fabricate all mating parts to extremely close tolerances, which becomes increasingly expensive as tolerances are reduced. Moreover, given the temperature ranges to which the parts are exposed before, during and after operation, and the resultant thermal expansion and contraction of the parts, such close tolerances would likely result in interference between mating parts and corresponding component wear and other damage. Accordingly, gas turbine engine designers have devoted significant effort to developing effective air seals, and particularly seals composed of abradable materials. See, e.g., U.S. Pat. Nos. 4,936,745 to Vine et al. and 5,706,231 to Nissley et al., which are assigned to the assignee of the present invention and expressly incorporated by reference herein. Such seals require a balance of several properties including abradability upon being contacted by a rotating blade tip, erosion resistance, durability, thermal expansion balanced with that of the underlying material, and relative ease and reasonable cost of manufacture. See, e.g., U.S. Pat. No. 5,536,022 to Sileo, which is also assigned to the assignee of the present invention and expressly incorporated by reference herein. To the extent that the seal is employed at elevated temperatures, the seal material must also be stable over a relatively wide temperature range and thermally insulate the underlying substrate.
A typical abradable air seal is described in U.S. Pat. No. 4,936,745 to Vine et al. The seal includes a metallic bond coat, e.g., a MCrAlY or aluminide bond coat, and a porous ceramic abradable layer, such as yttria stabilized zirconia (YSZ) having a controlled amount of porosity. Due to the porous, relatively soft structure of the YSZ layer, the material erodes not only upon contact by rotating blade tips, but also upon exposure to gasses at elevated velocities, pressures and temperatures, and upon contact by any particulate material entrained in the gasses. Erosion of the abradable material reduces thickness of the layer, which results in a gap and leakage between the blade tips and the case, and the vane tips and the disks, as well and also reduces the thermal insulation provided by the layer. To the extent that erosion extends through the ceramic layer, the underlying substrate is thermally protected only by the bond coat and whatever cooling air is provided to the seal. Accordingly, designers must either provide cooling for the seal, in an amount corresponding to an assumed absence of the ceramic material, or specify that the related structure, e.g., the seal, be removed and the ceramic material replaced after a relatively short service life. While known abradable materials provide effective sealing, it is desirable to provide seals having enhanced erosion resistance and correspondingly enhanced service lives.
It is an object of the present invention to provide a seal with enhanced erosion resistance, which maintains good abradability.
It is a further object of the present invention to provide a seal which provides significant thermal insulation to the underlying seal substrate, even if an abradable portion of the seal erodes or fails.
According to the present invention, a seal is disclosed which has particular use in a gas turbine engine. For example, the seal may serve as an air seal in compressor or turbine sections of the engine, and cooperate with a set of rotating blades of a section.
The seal includes a seal substrate or shoe, which is typically formed from a superalloy material. An adherent alumina layer of the seal is formed over the over the substrate, and may be formed as part of a bond coat that is a MCrAlY bond coat or an aluminide bond coat.
A relatively dense and erosion resistant ceramic layer is applied over the alumina layer. The seal also includes a porous abradable ceramic layer applied over a portion of the erosion resistant ceramic layer, and corresponds to an area in which the seal interacts with a cooperating part, such as a set of rotating turbine blades. The abradable ceramic layer thus cooperates with the blade tips to provide the sealing.
An advantage of the present invention is that the seal incorporates abradable material only where the seal cooperates with a mating component, such as the blade tips, to provide good sealing between the blades and the seal. An underlying layer of dense ceramic material provides enhanced erosion resistance and durability in addition to the thermal insulting capability of the ceramic material.
Additional advantages will become apparent to those skilled in the art in view of the following detailed description and attached drawings.